The protozoan parasite Toxoplasma gondii infects all nucleated cells and establishes life-long chronic infections in virtually any warm-blooded vertebrate. Eliminating the ability of this parasite to establish chronic infections in humans and animals is central to controlling its pathogenesis, however, there is currently no human vaccine or drug capable of doing this. Our lab has identified a large superfamily of >182 SRS protein adhesins that are essential for 1) entry into host cells and 2) regulating host immunity in order to establish chronic infections. The SRS proteins are expressed in a development-specific manner, and we have shown by gene-knockout studies that four of these antigens expressed by the tachyzoite stage are critical virulence factors: SRS29B, SRS29C, SRS34A, and SRS57. SRS57 is a pivotal adhesin required for establishing infection, whereas SRS29B, SRS29C, and SRS34A are primarily immunomodulating factors that elicit strong immunity in all infected hosts. We recently produced parasite strains deficient in the expression of SRS29B, SRS34A, SRS29C, and SRS57. Srs29B- strains were less pathogenic, infected mice had lower parasite burdens, their thymus exhibited only partial involution, and these mice produced substantially less IFN-&#947;, had reduced ileitis, in contrast to wild type parasite infections. Our results suggest that SRS29B is a critical virulence factor and that mice die from parasite burden and severe immune pathology dependent on SRS29B expression. Current investigations are testing whether collapse of regulatory T and B cells during acute ileitis is dependent on SRS29B. Srs34A- strains possessed an invasion defect, and were less virulent in murine infections. Previously, we showed that mouse virulent Toxoplasma strains only poorly express SRS29C, whereas all avirulent strains are high expressors. We therefore tested whether SRS29C expression level was sufficient to alter the mouse virulence phenotype. When expressed transgenically in a virulent strain at levels equivalent to those found in avirulent strains, the SRS29C transgenic strains were highly attenuated. These data suggest that SRS29C is a pivotal virulence factor and that its expression level is a critical determinant governing the outcome of infection. Importantly, we identified that SRS29C expression altered the levels of bioactive IL-1&#946; and IL-18 during infection of murine BMDMs and mice, respectively. To investigate whether Toxoplasma is capable of activating the inflammasome sensors NLRP1 and/or NLRP3, or if it can affect the secretion of the caspase-1-dependent proinflammatory cytokines IL-1&#946; and/or IL-18, we infected mice deficient in NLRP3, NLRP1abc, caspase-1/11, or the inflammasome adaptor protein ASC. Infection of these mice led to decreased levels of circulating IL-18, increased parasite replication, and death. Using mice deficient in IL-18 and IL-18R, we showed that this cytokine plays an important role limiting parasite replication to promote murine survival. We are currently investigating how expression of SRS29C alters IL-1&#946; and IL-18 expression during Toxoplasma infection of mice. Understanding the structural basis and the type of immunity induced during these natural infections should lay the foundation for therapeutic interventions, either prophylactic or vaccine-based, to limit infectivity and induce sterilizing immunity against this widespread zoonotic pathogen.